Compressor impeller cast from al alloy and method for producing same

ABSTRACT

The present invention provides: a compressor impeller that cast from an aluminum alloy, has superior production characteristics, and exhibits stable high-temperature strength when used at temperatures around 200° C.; and a method for producing same. The compressor impeller that is cast from an Al alloy is provided with a boss section, a plurality of vane sections and a disc section; the boss section, the plurality of vane sections and the disc section excluding the end section comprise an Al alloy comprising a predetermined metal composition; and at the end section of the disc section, there are at least 10,000/mm 2  of intermetallic compounds having a circle-equivalent diameter of 1-6 μm, and there are no greater than 500/mm 2  of intermetallic compounds having a circle-equivalent diameter exceeding 6 μm.

TECHNICAL FIELD

The present invention relates to a compressor impeller cast fromaluminum alloy for use in turbochargers of the internal combustionengines of automobiles and ships, and to a method for producing same.

BACKGROUND ART

The turbochargers used for the internal combustion engines ofautomobiles and ships include a compressor impeller that compresses andsupplies air into the internal combustion engine by rotating at highspeed. The compressor impeller can reach temperatures as high as about150° C. during its high-speed rotation, and receives high stress, suchas the torsional stress from the rotating shaft, and the centrifugalforce, near the center of rotation, particularly at the disc section.

Various materials are used for the compressor impeller according to therequired performance of the turbocharger. Hot forged materials of analuminum alloy machined into an impeller shape are typically used inlarge-scale applications such as ships. Mass production efficiency andcosts are more important in relatively smaller applications such as inautomobiles (e.g., cars, and trucks), and boats. Such applicationscommonly use easily castable aluminum alloys of primarily siliconadditive such as JIS-AC4CH (Al—7% Si—0.3% Mg alloy), ASTM-354.0 (Al—9%Si—1.8% Cu—0.5% Mg alloy), and ASTM-C355.0 (Al—5% Si—1.3% Cu—0.5% Mgalloy) of desirable castability. These materials are then cast with aplaster mold by using techniques such as low-pressure casting, vacuumcasting, and gravity casting, and are strengthened by a solutiontreatment or an aging treatment before use. A basic method of suchprocedures is disclosed in detail in Patent Document 1.

Lately, the need for high-speed turbochargers has increased with theincrease in the demand for higher compression ratios of air necessitatedby smaller engines, higher output, and increased exhaust recirculation.However, faster rotation speeds increase the amount of heat generated byair compression, and at the same time increase the temperature of theexhaust turbine impeller. The temperature of the compressor impeller isincreased by a heat transfer due to heat generation. It has been foundthat conventional compressor impellers made of easily castable aluminumalloys of primarily silicon additive tend to cause problems such asdeformation and fatigue failure during use, and fail to keep rotatingnormally. Specifically, these existing compressor impellers have anoperating temperature of at most about 150° C., and there is a strongneed for the development of a compressor impeller that can withstand anoperating temperature of about 200° C. to meet the demand for high speedrotations.

It may be possible to use an aluminum alloy composition of moredesirable high-temperature strength, for example, such as JIS-AC1B(Al—5% Cu—0.3% Mg alloy). However, as described in Patent Document 2,the problem of the alloy such as JIS-AC1B is that the molten metal lacksdesirable fluidity, and tends to cause misruns (underfilling) of themolten metal in thin section of vane sections when used to make articlesthat have complex shapes and thin vane sections such as in compressorimpellers.

Patent Document 2 addresses this problem by proposing a method that usesan Al—Si easily castable alloy such as AC4CH for the vane section forwhich misruns of a molten metal are of concern, and an Al—Cuhigh-strength alloy such as AC1B for the boss and disk sections that areconnected to the rotating shaft and thus require strength. These arecoalesced by being poured in two separate sections to form a compressorimpeller.

Patent Document 3 proposes a method that uses an alloy of desirablecastability for the vane section, and in which a strengthened compositematerial prepared by impregnating a strengthening material such as a25%-B (boron) aluminum whisker with aluminum is used for the stressedboss section and the central section of the disk section. These are thenjoined to each other to form a compressor impeller.

Patent Document 4 proposes a method in which a vane section and a bosssection (and a disk section) are joined to each other by frictionwelding. However, methods such as this that use different materials fordifferent sections are problematic in terms of productivity and cost,and are currently not usable in industrial applications.

Patent Document 5 addresses the problem of using different materials byproposing a compressor impeller that can be cast from a single alloy,specifically an Al—Cu—Mg-base alloy for which the additive elements andthe combination range of these elements are optimized. The resultingcompressor impeller has a proof stress value of 250 MPa or more at 180°C. Patent Document 6 proposes improving the casting yield by controllingthe crystal grain size of an Al—Cu—Mg-base alloy through optimization ofthe additive elements and the combination range of these elements. Thecompressor impeller has a proof stress value of 260 MPa or more at 200°C.

However, a problem remains that the products of the single alloy castingusing the Al—Cu—Mg-base alloy still need to stably withstand hightemperatures in the vicinity of 200° C. over extended time periods ifthese were to be used for ever faster turbochargers. Another unsolvedproblem is that the casting yield needs to be improved for stableproduction.

CITATION LIST Patent Document

Patent Document 1: U.S. Pat. No. 4,556,528

Patent Document 2: JP-A-10-58119

Patent Document 3: JP-A-10-212967

Patent Document 4: JP-A-11-343858

Patent Document 5: JP-A-2005-206927

Patent Document 6: JP-A-2012-25986

SUMMARY Technical Problem

The present invention has been made in view of the foregoing problems,and it is an object of the present invention to provide a compressorimpeller cast from an aluminum alloy (hereinafter, “Al alloy”) thatremains stably strong over extended time periods even under operatingtemperatures of about 200° C., and that excels in productivity. Theinvention is also intended to provide a method for producing suchimpellers.

Solution to Problem

With regard to the foregoing problems, the present inventors focused onthe disc section of compressor impellers that receives high stress, andfound that the strength of the disc section greatly improves when theintermetallic compounds at the end section of the disc section arefinely dispersed. The present inventors also diligently studied a methodof production for finely dispersing intermetallic compounds, and foundthat refining primary phase aluminum crystal grains is important forfine dispersal of intermetallic compounds, and that controlling thecooling rate of Al alloy molten metal, and controlling the distributionof refined particles in a compressor impeller are important to achievethis. The present invention was completed on the basis of thesefindings.

In claim 1, the present invention is directed to a compressor impellercast from an Al alloy comprising a boss section, a plurality of vanesections and a disc section,

wherein the boss section, the plurality of vane sections, and the discsection excluding an end section comprise an Al alloy comprising Cu: 1.4to 3.2 mass %, Mg: 1.0 to 2.0 mass %, Ni: 0.5 to 2.0 mass %, Fe: 0.5 to2.0 mass %, Ti: 0.01 to 0.35 mass %, and B: 0.002 to 0.070 mass % and abalance of Al and unavoidable impurities,

wherein the end section of the disc section comprises an Al alloycomprising Cu: 1.4 to 3.2 mass %, Mg: 1.0 to 2.0 mass %, Ni: 0.5 to 2.0mass %, Fe: 0.5 to 2.0 mass %, Ti: 0.005 to 0.175 mass %, and B: 0.001to 0.035 mass % and a balance of Al and unavoidable impurities, and

wherein at least 10000/mm² of intermetallic compounds having acircle-equivalent diameter of 1 to 6 μm, and no greater than 500/mm² ofintermetallic compounds having a circle-equivalent diameter exceeding 6μm exist in the end section of the disc section.

In claim 2 of the present invention according to claim 1, the compressorimpeller is for use in large-scale applications including ships, and theboss section has a height of 200 to 80 mm, the disc section has adiameter of 300 to 100 mm and the vane sections have a height of 180 to60 mm with 30 to 10 vanes measuring 4.0 to 0.4 mm in thickness at a vanetip.

In claim 3 of the present invention according to claim 1, the compressorimpeller is for use in small-scale applications including automobiles,and the boss section has a height of 100 to 20 mm, the disc section hasa diameter of 120 to 25 mm, and the vane sections have a height of 90 to5 mm with 20 to 4 vanes measuring 3.0 to 0.1 mm in thickness at a vanetip.

In claim 4, the present invention is directed to a method for producinga compressor impeller cast from an Al alloy, comprising:

a molten metal preparation step of preparing a 720 to 780° C. Al alloymolten metal comprising Cu: 1.4 to 3.2 mass %, Mg: 1.0 to 2.0 mass %,Ni: 0.5 to 2.0 mass %, Fe: 0.5 to 2.0 mass % and a balance of Al andunavoidable impurities, and adding a refining agent to the Al alloymolten metal to incorporate Ti: 0.01 to 0.35 mass % and B: 0.002 to0.070 mass % in the alloy composition;

a casting step of casting an Al alloy casting by pressure castingwhereby the Al alloy molten metal prepared is injected through a moltenmetal inlet into a space having a product shape configured from aplaster mold having the molten metal inlet at the bottom of the plastermold, and a 100 to 250° C. chill disposed on a surface that contactswith an impeller disc surface, the space being formed by disposing theplaster mold and the chill so that the chill is at upper position andthe plaster mold is at below position, and an inflow rate at the moltenmetal inlet into the space being 0.12 to 1.00 m/s;

a solution treatment step of solution treating by subjecting the Alalloy casting to a solution treatment; and

an aging treatment step of aging treating by subjecting the Al alloycasting to aging after the solution treatment.

In claim 5 of the present invention according to claim 4, the endsection of the disc section has a cooling rate of 0.1 to 200° C./s inthe casting step.

In claim 6 of the present invention according to claim 4 or claim 5, theAl alloy casting is heat treated for 2 hours or more at a temperature 5to 25° C. below the solidus temperature of the Al alloy in the solutiontreatment step, and the solution-treated Al alloy casting is subjectedto a heat treatment at 180 to 230° C. for 3 to 30 hours in the agingtreatment step.

The present invention can provide an aluminum alloy cast impeller forcompressors that shows stable high-temperature strength even in a hightemperature range in the vicinity of 200° C. over extended time periods,and that has excellent productivity such as casting yield.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view representing an exemplary structure of anAl alloy cast impeller for compressors according to the presentinvention.

FIG. 2 is an explanatory diagram representing the measurement areas ofintermetallic compound distribution in the Al alloy cast impeller forcompressors according to the present invention.

FIG. 3 is an explanatory diagram representing how a plaster mold and achill are disposed, and the pour direction of molten metal upwardlypoured into a space configured from the plaster mold and the chill in amethod for producing the Al alloy cast impeller for compressorsaccording to the present invention.

FIG. 4 is an explanatory diagram representing how the plaster mold andthe chill are disposed, and the pour direction of molten metal laterallypoured into the space configured from the plaster mold and the chill.

FIG. 5 is an explanatory diagram representing how the plaster mold andthe chill are disposed, and the pour direction of molten metaldownwardly poured into the space configured from the plaster mold andthe chill.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention is described below in detail.

A. Features of Aluminum Alloy Cast Impeller for Compressors According tothe Present Invention

After a series of various experimental studies conducted to solve theforegoing problems, the present inventors found that a compressorimpeller that stably maintains excellent high-temperature strength overextended time periods without involving damage to the disc section canbe obtained by optimizing the size and the surface density ofintermetallic compounds at the end section of the disc section of thecompressor impeller, even when the compressor impeller is used underhigh temperatures of about 200° C. It was also found that the size andthe surface density of intermetallic compounds at the end section of thedisc section of the compressor impeller can be optimized, and that thecasting yield can improve from conventional yields by optimizing the Alalloy composition, controlling the cooling rate of casting throughadjustments of molten metal temperature and chill temperature, andcontrolling the pour rate of molten metal into a compressor impellermold.

As used herein, “stably maintain desirable high-temperature strengthover extended time periods” means that deformation and fatigue failuredo not occur over extended time periods even under operatingtemperatures of about 200° C. Specifically, it means that no damageoccurs in a turbo assembly durability test conducted at 200° C. for150,000 rpm×200 hours.

B. Shape of Al Alloy Cast Impeller for Compressors

FIG. 1 shows an example of the shape of the aluminum alloy cast impellerfor compressors (hereinafter, simply referred to “compressor impeller”)according to the present embodiment. A compressor impeller 1 includes arotational center shaft (boss section) 2, a disk section 3 continuousfrom the boss section 2, and a plurality of thin vanes 4 projectingoutwardly from the disk section 3. The compressor impeller 1 reaches atemperature as high as about 200° C. during high-speed rotation, andreceives high repeated stress along a vertical direction, particularlyat the disk section end.

C. Al Alloy Composition

The composition of the Al alloy used in the present invention isdescribed below along with the reasons for limiting the Al alloycomponents.

C-1. Cu, Mg:

Cu and Mg dissolve into the Al matrix and show an effect that amechanical strength is improved by the solid solution strengthening. Byexisting together, Cu and Mg also contribute to improving strengththrough precipitation strengthening such as by Al₂Cu, and Al₂CuMg.Because these two elements widen the solidification temperature range,excess addition of these elements is detrimental to castability.

When the Cu content is less than 1.4 mass % (hereinafter, simplyreferred to “%”), and/or Mg content is less than 1.00%, the requiredmechanical strength at high temperatures of around 200° C. may not beobtained. On the other hand, when the Cu content is above 3.2%, and/orMg content is in excess of 2.0%, the castability of the compressorimpeller is impaired, and may cause an underfill as the molten metalfails to sufficiently run into the vane end section in particular. Forthese reasons, the Cu content should preferably be 1.4 to 3.2%, and theMg content should preferably be 1.0 to 2.0%. The Cu content is morepreferably 1.7 to 2.8%, and the Mg content is more preferably 1.3 to1.8% in terms of surely preventing defects such as deformation duringuse, and practically preventing generation of an underfill duringcasting and obtaining an industrially preferable yield.

C-2. Ni, Fe:

Ni and Fe form an intermetallic compound with Al, and disperse into theAl matrix to improve the high-temperature strength of the Al alloy. Tothis end, the Ni content should be 0.5% or more, and the Fe contentshould be 0.5% or more. However, when contained in excess, theseelements not only coarsen the intermetallic compound, but reduce theamount of the solid solution Cu in the Al matrix, and lower strength byforming Cu₂FeAl₇ and Cu₃NiAl₆ at high temperatures. The presence ofcoarsened intermetallic compounds at the end section of the disc sectioncauses damage to the compressor impeller as the intermetallic compoundsbecome a starting point of damage under the repeatedly applied stress onthe end section of the disc section, as will be described later. It istherefore preferable to contain Ni in a content of 2.0% or less, and Fein a content of 2.0% or less. Taken together, the Ni content shouldpreferably be 0.5 to 2.0%, and the Fe content should preferably be 0.5to 2.0%. Preferably, the Ni content is 0.5 to 1.4%, and the Fe contentis 0.7 to 1.5%. The lower limits of these preferred ranges are providedas indications for stably mass producing products in industrial settingsby reducing production variation, whereas the upper limits areindications above which the effects will be saturated, and the elementswill be wasted.

C-3. Ti, B:

Ti and B have the effect to inhibit the growth of primary phase aluminumcrystal grains during casting, and are added to reduce the size of thesolid structure in the casting, and improve the supply and the run ofmolten metal. These effects may not be sufficiently obtained when the Ticontent is less than 0.01%, and/or the B content is less than 0.002% inthe boss section, the vane sections, and the disc section other than theend section, or when the Ti content is less than 0.005%, and/or the Bcontent is less than 0.001% in the end section of the disc section. Onthe other hand, the refining agent particles aggregate, and fatiguecracking occurs from the aggregates when the Ti content exceeds 0.35%,and/or the B content exceeds 0.070% in the boss section, the vanesections, and the disc section other than the end section, or when theTi content exceeds 0.175%, and/or the B content exceeds 0.035% in theend section of the disc section. When contained in excess of 0.35% inthese sections, Ti forms coarse intermetallic compounds of several tento several hundred micrometers with Al. Such coarse intermetalliccompounds become a starting point of fatigue cracking, and lowers thereliability of the compressor impeller. For these reasons, the Ticontent should be 0.01 to 0.35%, and the B content should be 0.002 to0.070%, preferably the Ti content is 0.15 to 0.30%, and the B content is0.003 to 0.060% in the boss section, the vane sections, and the discsection other than the end section. At the end section of the discsection, the Ti content should be 0.005 to 0.175%, and the B contentshould be 0.001 to 0.035%, preferably the Ti content is 0.010 to 0.165%,and the B content is 0.002 to 0.033%.

The Al alloy may contain unavoidable impurities, such as about 0.3% orless of Si, and about 0.2% or less of each of Zn, Mn, and Cr. Theseunavoidable impurities are acceptable because these do not affect thecharacteristics of the compressor impeller.

D. Intermetallic Compound

The aluminum alloy used in the present invention is cast into a shape ofthe compressor impeller with a plaster mold by pressure casting(low-pressure casting, vacuum casting, or differential pressure casting)according to a conventional Al—Si aluminum alloy casting producingmethod.

In the pressure casting using a plaster mold, producing conditions needto be controlled with regard to the distribution of the inter metalliccompounds inside each casting in such a manner that the number ofintermetallic compounds having a circle-equivalent diameter of 1 to 6 μmis at least 10000/mm² at the end section of the disc section, and thatthe number of intermetallic compounds having a circle-equivalentdiameter exceeding 6 μm is no greater than 500/mm² at the end section ofthe disc section.

After studying the damaging behavior of the compressor impeller, thepresent inventors found that end section of the disc section receivesrepeatedly occurring high normal stress due to acceleration anddeceleration of compressor impeller rotation, and becomes damaged as thecoarse intermetallic compounds having a circle-equivalent diameterexceeding 6 μm at the end section of the disc section become a startingpoint of cracking and develop cracks. After further studies, it was alsofound that cracking originating from such intermetallic compounds, andpropagation of cracks, when present, can be inhibited when theintermetallic compounds having a circle-equivalent diameter exceeding 6μm at the end section of the disc section has a surface density of nogreater than 500/mm². It was also found that generation of the coarseintermetallic compounds can be inhibited when the number ofintermetallic compounds having a circle-equivalent diameter of 1 to 6 μmat the end section of the disc section is at least 10000/mm².

The generated amounts of intermetallic compounds depend on thecomposition under typical casting conditions. Here, “under typicalcasting conditions” means under the cooling rate of low-pressurecasting, specifically 0.1 to 200° C./s. Studies of intermetalliccompound changes after a post-casting heat treatment revealed that theheat treatment does not have a large effect on the size of theintermetallic compounds generated during casting. By generating largeamounts of fine intermetallic compounds during casting, it then becomespossible to inhibit generation of coarse intermetallic compounds in thesubsequent heat treatment.

The reason that the intermetallic compounds having a circle-equivalentdiameter of 1 to 6 μm are made to have a surface density of at least10000/mm² is as follows. Intermetallic compounds having acircle-equivalent diameter of less than 1 μm do not affect thecompressor impeller strength. When the surface density of theintermetallic compounds having a circle-equivalent diameter of 1 to 6 μmis less than 10000/mm², the generation of intermetallic compounds havinga circle-equivalent diameter exceeding 6 μm accelerates, and crackingoccurs from the generated intermetallic compounds having acircle-equivalent diameter exceeding 6 μm. The upper limit of thesurface density is not particularly limited, and is determined by thecomposition of the Al alloy, and the producing conditions. In thepresent invention, the upper limit is 30000/mm².

The reason that intermetallic compounds having a circle-equivalentdiameter exceeding 6 μm is made to have a surface density of no greaterthan 500/mm² is as follows. Intermetallic compounds having acircle-equivalent diameter exceeding 6 μm are of interest for the reasondescribed above. When the surface density exceeds 500/mm², crackingpropagates as the distances between the intermetallic compounds becomeshorter. The lower limit of the surface density is not particularlylimited, and depends on the composition of the Al alloy, and theproducing conditions. In the present invention, the lower limit ispreferably 100/mm², most preferably 0/mm².

Examples of the intermetallic compounds generated in the presentinvention include Al—Fe—Ni—Cu, Al—Fe—Cu—Ni—Mg, Al—Cu—Mg, Al—Cu,Al—Cu—Mg—Si, Al—Cu—Fe, Al—Ni, Al—Mg, and Mg—Si intermetallic compounds.The circle equivalent size of the generated intermetallic compounds hasa distribution in a range of from 0.1 to 20.0 μm, though it depends onthe composition of the Al alloy, and the producing conditions. As usedherein, “circle equivalent size” means “circle equivalent diameter.”

E. Controlling Amounts of Refining Agent Components in End Section ofDisc Section

Controlling the size of primary phase aluminum crystal grains isimportant in controlling the distribution of the intermetalliccompounds. This is because the intermetallic compounds are generated atthe grain boundary of primary phase aluminum crystal grains. Factorsthat are important in controlling the size of primary phase aluminumcrystal grains are amounts of the refining agent components, and thecooling rate which will be described later.

The appropriate amounts of the Ti and B components in the end section ofthe disc section are 0.005 to 0.175% for Ti, and 0.001 to 0.035% for B.In order to achieve these refining agent contents, a refining agentcomprised of Al, Ti, and B is added in the molten metal preparation stepto make the amounts of the Ti and B components 0.01 to 0.35% and 0.002to 0.070%, respectively, in the aluminum alloy molten metal after themolten metal preparation step. The product-shape space configured fromthe plaster mold and the chill is formed by vertically disposing thesemembers so that the chill is higher in position than the plaster moldhaving a molten metal inlet at the bottom. The prepared Al alloy moltenmetal is poured into the space through the molten metal inlet. Themolten metal is poured into the space at a pour rate of 0.12 to 1.00 m/sat the molten metal inlet. The casting step of pressure casting the Alalloy casting by injecting the prepared molten metal into the space isadapted to satisfy the foregoing requirements.

One of the reasons for specifying the foregoing requirements is thatonly a section of the refining agent amount in the molten metalpreparation step reaches end section of the disc section under theeffect of the molten metal flow inside the space. The present inventorshave confirmed that this is due to the refining agent particles failingto follow the moving molten metal during pressure casting in accordancewith the law of inertia. The following describes this in detail.

The refining agent comprised of Al, Ti, and B is added in the moltenmetal preparation step. When the amount of the Ti component is less than0.01%, and/or the amount of the B component is less than 0.002% in thealuminum alloy molten metal after the molten metal preparation step, theprimary phase aluminum crystal grains coarsen, which, in turn, coarsensthe intermetallic compounds at the grain boundary to lower the strengthof the Al alloy material. When the amount of the Ti component exceeds0.35%, and/or the amount of the B component exceeds 0.070% in thealuminum alloy molten metal after the molten metal preparation step,coarse TiB₂ aggregates are generated, and become a starting point offracture. The amounts of the Ti and B components in the aluminum alloymolten metal after adding the refining agent in the molten metalpreparation step are therefore 0.01 to 0.35% for Ti, and 0.002 to 0.070%for B.

The product-shape space configured from the plaster mold and the chillis described below. As shown in FIG. 3, the product-shape space 10configured from a plaster mold 7 and a chill 6 is formed by verticallydisposing these members so that the chill 6 is higher in position thanthe plaster mold 7. A molten metal inlet 8 for pouring molten metal intothe space 10 is provided at the bottom on the plaster mold 7 side.Molten metal is charged into the space 10 by being poured into thespace, from the bottom to the top of the figure, through the moltenmetal inlet 8 along the molten metal pour direction 9.

Without the space 10 configured as above, balancing during high-speedrotations of the compressor impeller suffers as a result of thenonuniform solidification occurring in the circumferential direction ofthe compressor impeller, and operation of the device becomes limited asthe casting machine becomes complex. For example, as shown FIG. 4, whenthe plaster mold 7 and the chill 6 are horizontally disposed to form theproduct-shape space 10 configured from these members, the molten metalis charged into the space 10 by being laterally poured into the space10, from the right to left in the figure, through the molten metal inlet8 along the molten metal pour direction 9. In this case, the moltenmetal poured into the space 10 solidifies as it fills the space 10 fromthe bottom to the top. The molten metal thus solidifies earlier in thecircumferential section at the bottom side of the space 10 than in theupper circumferential section in the circumferential direction of thecompressor impeller, and fails to produce a uniform solid state in thecircumferential direction. The nonuniform solidification in thecircumferential direction of the compressor impeller causes a bend inthe shaft section, and produces an imbalance during high-speedrotations.

On the other hand, for example, as shown in FIG. 5, when theproduct-shape space 10 configured from the plaster mold 7 and the chill6 is formed by vertically disposing these members so that the plastermold 7 is higher in position than the chill 6, the molten metal ischarged into the space 10 by being poured into the space 10 through themolten metal inlet 8 along the molten metal pour direction 9 from thetop to the bottom of the figure. This complicates the piping of thepipes (stalks) used to charge the molten metal in a furnace into thespace 10. Specifically, a stalk needs to be piped to create a verticallydownward flow of the molten metal vertically discharged upward frominside of a furnace in pressure casting. This inevitably complicates thestalk piping. Further, because the stalk distance increases, a loweredmolten metal temperature or an increased pressure loss of the moltenmetal flow makes the casting difficult.

When the product-shape space 10 is formed by disposing the plaster mold7 and the chill 6 as shown in FIG. 3, the nonuniform solid state incircumferential direction seen in the configuration of FIG. 4, or thecomplex stalks, decrease of molten metal temperature, and increase inthe pressure loss of the molten metal flow seen in the configuration ofFIG. 5 do not occur.

The pour rate of molten metal at the molten metal inlet is an importantfactor in controlling the refining agent content at the disc sectionend. As described above, the content of the refining agent particlescomprised of Ti and B reaching the end section of the disc sectiondecreases from the content of refining agent particles in the moltenmetal preparation step. This is due to the refining agent particlesfailing to follow the moving molten metal during pressure casting inaccordance with the law of inertia. When the pour rate of molten metalat the molten metal inlet exceeds 1.00 m/s, the excessively fast pourrate of the molten metal makes it even more difficult for the inertialmovement of the refining agent particles to follow the pour rate of themolten metal, and the refining agent particles cannot reach the endsection of the disc section in amounts that are necessary for grainrefining. On the other hand, when the pour rate of the molten metal atthe molten metal inlet is less than 0.12 m/s, the excessively slow pourrate of the molten metal increases the time it takes for the moltenmetal to reach the plaster mold through the stalk. This lowers themolten metal temperature, and causes solidification failure. The pourrate of molten metal at the molten metal inlet is preferably 0.20 to0.85 m/s.

F. Controlling Cooling Rate at End Section of Disc Section

In order to obtain the intermetallic compound distribution above, thecooling rate at the end section of the disc section of the compressorwheel needs to be controlled. Specifically, the molten metal temperatureis controlled between 720 and 780° C., and the temperature of the chill(chill plate) disposed on the surface that contacts the compressor discsurface is controlled between 100 and 250° C. The cooling rate at theend section of the disc section is adjusted in a preferred range of 0.1to 200° C./s by specifying the molten metal temperature and the chilltemperature as above. When the cooling rate is less than 0.1° C./s, theprimary phase aluminum crystal grain coarsens, which coarsens theintermetallic compounds generated at the grain boundary. Further,shrinkage cavity occurs, and productivity suffers as the cooling ratedecreases. On the other hand, when the cooling rate is above 200° C./s,early solidification occurs inside the product-shape space. This causesmisruns, and the intended product shape cannot be ensured. The coolingrate at the end section of the disc section is further preferably 3 to150° C./s.

When the molten metal temperature is below 720° C., the injected moltenmetal solidifies early inside the product-shape space. This causesmisruns, and the intended product shape cannot be ensured. On the otherhand, with a molten metal temperature above 780° C., the molten metalprogressively undergoes oxidation, and increased porosity numbers due tohydrogen gas absorption, and increased oxides impair the quality of themolten metal. This makes it difficult to ensure product strength.

When the chill temperature is below 100° C., solidification proceeds atan excessive rate, and misruns occur. On the other hand, when the chilltemperature is above 250° C., solidification from the chill slows down,and the slow cooling rate coarsens the primary phase aluminum crystalgrains, which coarsens the intermetallic compounds generated at thegrain boundary. Further, a chill temperature above 250° C. causes burrdefects, which occur when the molten metal enters between the plastermold and the chill.

In the present invention, it is preferable to control the preheatingtemperature of the plaster mold between 200 and 350° C., though thetemperature is not particularly limited. When the preheating temperatureof the plaster mold is less than 200° C., solidification takes placebefore the charged molten metal fills the mold end. This causes misruns,and the intended product shape cannot be ensured. On the other hand,when the preheating temperature of the plaster mold exceeds 350° C., thesolidification slows down inside the plaster mold, and a shrinkagecavity failure occurs.

The chill material is preferably copper or a copper alloy, which hashigh thermal conductivity. However, materials such as steel, andstainless steel also may be used. Preferably, the chill temperature isadjusted by using a mechanism by which superheating in the casting isreduced with a coolant such as water passed inside the chill.

G. Producing Method

A method for producing the Al alloy cast impeller for compressorsaccording to the present invention is described below. The producingmethod includes a molten metal adjusting step, a casting step, and aheat treatment step.

Molten Metal Preparation Step:

Each component element is melted under heat to make the Al alloycomposition above by using an ordinary method, and molten metalprocesses such as processing of dehydrogenated gas, and removal ofinclusions are performed. The temperature is adjusted to make the finalmolten metal temperature 720 to 780° C. The hydrogen gas amount in themolten metal is also adjusted. A rotary gas blower is used to adjust thehydrogen gas amount in the molten metal. However, the method is notlimited to this.

Casting Step:

In the casting step, the molten metal adjusted to 720 to 780° C. is castinto a shape of the compressor impeller by pressure casting using aplaster mold. As described above, the temperature of the chill disposedon the surface that contacts the disc surface is adjusted to 100° C. to250° C. As for the product-shape space configured from the plaster moldand the chill, the product-shape space configured from the plaster moldand the chill is formed by vertically disposing these members so thatthe chill is higher in position than the plaster mold, as shown in FIG.3. The molten metal inlet through which the molten metal is poured intothe space in molten metal pour direction is provided at the bottom onthe plaster mold 7 side. The pour rate of the molten metal into thespace through the molten metal inlet is adjusted to 0.12 to 1.00 m/s.The Al alloy casting is cast by pressure casting whereby the prepared Alalloy molten metal is injected into the space as above.

Heat Treatment Step:

The Al alloy casting is subjected to a heat treatment step. The heattreatment step includes a solution treatment step and an aging treatmentstep. The heat treatment step can effectively take advantage of thesolid solution strengthening by Cu; the precipitation strengthening byCu and Mg; and the dispersion strengthening by the intermetalliccompounds formed by Al and Fe and by Al and Ni.

Solution Treatment Step:

The solution treatment is performed preferably in a temperature rangethat is 5 to 25° C. lower than the solidus temperature. In the preferredAl alloys for use in the present invention, a temperature range of 510to 530° C. represents such a temperature range that is 5 to 25° C. lowerthan the solidus temperature. The risk of melting the second phase ofcrystal grain boundaries increases, and it becomes difficult to ensurestrength at temperatures above the temperature range that is 5 to 25° C.lower than the solidus temperature. On the other hand, the elements donot diffuse sufficiently, and the solution treatment becomesinsufficient at temperatures below the temperature range that is 5 to25° C. lower than the solidus temperature. Preferably, the solutiontreatment is performed for at least 2 hours. The elements do not diffusesufficiently, and the solution treatment becomes insufficient when thesolution treatment is less than 2 hours. Considering mass production,the solution treatment by element diffusion is performed for preferably30 hours or less, though the treatment time is not particularly limitedas long as the solution treatment is performed for at least 2 hours.

Aging Treatment:

The aging treatment involves a heat treatment performed preferably at180 to 230° C. for 3 to 30 hours, more preferably 190 to 210° C. for 5to 20 hours. The precipitation strengthening for improving strength maybecome insufficient when the process temperature is below 180° C., orwhen the process time is less than 3 hours. On the other hand, theprecipitated phase formed may coarsen (overaging), and may fail toprovide a sufficient strengthening effect, and the solid solutionstrengthening capability of Cu weakens when the process temperatureexceeds 230° C., or when the process time exceeds 30 hours.

H. Shape of Compressor Wheel

The shape and the dimensions of the compressor impeller according to thepresent invention, and the number of vanes of the compressor impellerare not particularly limited, and the compressor impeller is applicableto many different applications, ranging from large-scale applicationssuch as ships to small-scale applications such as automobiles. Taking alarge scale application such as ships as an example, the boss sectionhas a height of 200 to 80 mm, preferably 180 to 100 mm, the disc sectionhas a diameter of 300 to 100 mm, preferably 260 to 120 mm, and the vanesections have a height of 180 to 60 mm, preferably 160 to 90 mm. Thethickness at the tip of the vane is 4.0 to 0.4 mm, preferably 3.0 to 0.6mm. The number of vanes is 30 to 10, preferably 26 to 12. In the case ofsmaller applications such as automobiles, the boss section has a heightof 100 to 20 mm, preferably 90 to 25 mm, the disc section has a diameterof 120 to 25 mm, preferably 100 to 30 mm, and the vane sections have aheight of 90 to 5 mm, preferably 80 to 8 mm. The thickness at the tip ofthe vane is 3.0 to 0.1 mm, preferably 2.0 to 0.2 mm. The number of vanesis 20 to 4, preferably 18 to 6.

Examples

The present invention is described below in greater detail usingExamples.

First Example (Present Examples 1 to 7, and Comparative Examples 1 to20)

Each Al alloy of the composition shown under the column “Components” inTable 1 was melted by using a common molten metal process, and themolten metal was adjusted to the temperature shown in Table 1 by amolten metal preparation step. In the molten metal preparation step, 150kg of the Al alloy of the composition shown under the column“Components” in Table 1 was melted to obtain a molten metal. Thereafter,a blow degassing process was performed by blowing argon gas into themolten metal for 30 minutes with a rotary gas blower operated at arotation speed of 400 rpm, and a gas flow rate of 2.5 Nm³/h. The wholemolten metal was held still for 1 hour to remove the slag. After theslag removal, a refining agent was added to the molten metal in themetal preparation step to make the amounts of Ti and B components asshown under the column “Amounts of refining agent components aftermolten metal preparation” in Table 1.

TABLE 1 Amounts of refining agent Casting conditions components Plas-Pour rate after Heat treatment conditions Molten ter of molten Moltenmolten metal Solution Aging metal Chill mold metal into metalpreparation treatment temp. × treatment Components (mass %) temp. temp.temp. mold pour (mass %) time temp. × time No. Cu Mg Ni Fe Al (° C.) (°C.) (° C.) (m/s) direction Ti B (° C. × hour) (° C. × hour) Present Ex.1 3.2 2.0 1.9 2.0 Remainder 760 210 206 0.15 Upward 0.10 0.020 530 × 8200 × 20 Present Ex. 2 3.1 1.9 1.4 1.5 excluding Ti 780 250 201 0.90Upward 0.20 0.040 Present Ex. 3 2.2 1.6 0.8 1.0 and B 760 110 267 0.35Upward 0.15 0.030 Present Ex. 4 1.6 1.4 0.6 0.7 740 220 250 0.12 Upward0.35 0.070 Present Ex. 5 2.6 1.6 0.8 1.1 750 130 240 1.00 Upward 0.130.020 Present Ex. 6 1.4 1.2 1.2 1.0 720 240 210 0.85 Upward 0.08 0.015Present Ex. 7 2.4 1.0 0.5 0.5 740 230 350 0.20 Upward 0.01 0.002 Com.Ex. 1 2.9 1.7 1.6 1.1 770 260 304 0.45 Upward 0.05 0.010 Com. Ex. 2 2.01.1 1.2 0.9 740 90 341 0.75 Upward 0.27 0.050 Com. Ex. 3 2.1 1.2 1.4 1.5710 160 319 0.80 Upward 0.21 0.040 Com. Ex. 4 2.5 1.3 1.7 1.3 790 200239 0.90 Upward 0.12 0.020 Com. Ex. 5 1.3 1.9 1.4 1.2 730 180 312 0.45Upward 0.07 0.012 Com. Ex. 6 2.8 0.9 1.1 1.4 750 190 330 0.30 Upward0.15 0.030 Com. Ex. 7 3.0 1.4 1.4 0.4 760 170 230 0.20 Upward 0.23 0.045Com. Ex. 8 2.9 1.3 0.4 1.7 770 200 235 0.30 Upward 0.18 0.035 Com. Ex. 92.6 1.4 0.9 1.2 765 210 197 0.50 Upward 0.01 0.001 Com. Ex. 10 3.3 1.81.1 1.2 740 185 316 0.80 Upward 0.23 0.045 Com. Ex. 11 2.5 2.1 0.9 1.1750 150 231 0.90 Upward 0.19 0.036 Com. Ex. 12 2.9 1.5 1.4 2.1 730 225279 0.30 Upward 0.26 0.050 Com. Ex. 13 2.2 1.6 2.1 1.2 760 190 289 0.80Upward 0.18 0.036 Com. Ex. 14 2.9 1.3 1.2 1.2 740 210 366 1.20 Upward0.38 0.075 Com. Ex. 15 2.8 1.1 1.7 0.9 750 220 260 1.50 Upward 0.020.003 Com. Ex. 16 3.0 1.6 0.8 1.0 740 190 296 0.90 Upward 0.37 0.072Com. Ex. 17 2.0 1.1 1.2 0.9 760 180 244 0.11 Upward 0.27 0.050 Com. Ex.18 2.9 1.3 1.7 1.2 730 240 231 1.01 Upward 0.23 0.015 Com. Ex. 19 2.51.3 1.1 1.2 740 170 348 0.75 Lateral 0.26 0.050 Com. Ex. 20 3.1 1.6 0.91.1 770 200 256 0.20 Downward 0.20 0.040

The Al alloy molten metal prepared in the molten metal preparation stepwas then subjected to low-pressure casting to produce an Al alloycasting, whereby the molten metal was pressure injected into apredetermined space configured from a plaster mold that had beenadjusted to 250° C., and a copper chill that had been adjusted to thetemperature shown in Table 1 and disposed on the surface that contactsthe impeller disc surface. Here, the molten metal was injected throughthe molten metal inlet provided at the bottom of the lower plaster mold(FIG. 3), the side of the side plaster mold (FIG. 4), or the top of theupper plaster mold (FIG. 5). The Al alloy casting was intended as aturbocharger compressor impeller for cars, and had a shape with a discsection measuring 40 mm in diameter, a boss section measuring 40 mm inheight, vane sections measuring 35 mm in height and having 12 vanes thatwere 0.3 mm in thickness at the vane tip. The molten metal was pressureinjected into the space configured from the plaster mold and the chillin the directions shown in Table 1 through the molten metal inlet at thepour rates shown in Table 1, and the pressure was maintained until thewhole Al alloy casting solidified.

The Al alloy casting was removed from the plaster mold, and subjected toa solution treatment at 530° C. for 8 hours, and thereafter to an agingtreatment at 200° C. for 20 hours. In this way, a sample Al alloy castimpeller for compressors was prepared.

The samples produced in the manner described above were each evaluatedwith respect to the surface density of intermetallic compounds having acircle equivalent size of 1 to 6 μm at the end section of the discsection, the surface density of intermetallic compounds having a circleequivalent size of more than 6 μm at the end section of the discsection, amounts of the refining agent components (Ti, B) at the endsection of the disc section, amounts of the refining agent components(Ti, B) in sections other than the end section of the disc section,high-temperature characteristics (durability test evaluation), andproductivity (casting yield evaluation), as follows.

1. Surface Density Measurement of Intermetallic Compounds

The samples were cut along the central axis to determine the size andthe surface density of intermetallic compounds at the end section of thedisc section. FIG. 2 shows a cross section on one side of the centralaxis 5 of the compressor impeller. The end section 31 of the discsection in the cross section was cut and polished, and imaged with anoptical microscope at 100× magnification. Here, the end section 31 ofthe disc section represents 20% of the disc section from thecircumference of the disc section of the compressor impeller to thecentral axis 5 along the radial direction. The image was fed to an imageanalyzer, and measured for the surface density of intermetalliccompounds having a circle equivalent size of 1 to 6 μm, and the surfacedensity of intermetallic compounds having a circle equivalent size ofmore than 6 μm. The measurements were made at arbitrarily selected 10measurement points, and the arithmetic mean value was calculated assurface density. Each measurement point had a view area of 1 mm². Theresults are presented in Table 2.

TABLE 2 Intermetallic compounds at end section of disc section Surfacedensity of Surface density of Amounts of refining intermetallicintermetallic Amounts of refining agent components compounds havingcompounds having agent components in sections other circle equivalentcircle equivalent size at end section of than end section of size of 1to 6 μm of more than 6 μm disc section disc section (Number of (Numberof Ti Ti intermetallic intermetallic content B content content B contentNo. compounds/mm²) compounds/mm²) (mass %) (mass %) (mass %) (mass %)Present Ex. 1 14546 250 0.047 0.009 0.10 0.019 Present Ex. 2 15460 3930.029 0.006 0.13 0.026 Present Ex. 3 29970 101 0.060 0.012 0.14 0.027Present Ex. 4 12318 469 0.175 0.035 0.35 0.070 Present Ex. 5 14948 3900.008 0.002 0.06 0.012 Present Ex. 6 15727 230 0.012 0.002 0.05 0.010Present Ex. 7 22814 110 0.005 0.001 0.01 0.002 Com. Ex. 1 9386 670 0.0170.003 0.04 0.008 Com. Ex. 2 24385 197 0.052 0.010 0.18 0.035 Com. Ex. 317715 227 0.038 0.008 0.14 0.028 Com. Ex. 4 8659 471 0.012 0.002 0.060.012 Com. Ex. 5 19094 207 0.018 0.004 0.05 0.010 Com. Ex. 6 19429 1990.063 0.013 0.14 0.028 Com. Ex. 7 10934 405 0.104 0.021 0.22 0.043 Com.Ex. 8 11170 397 0.072 0.014 0.16 0.032 Com. Ex. 9 7472 577 0.004 0.0000.00 0.001 Com. Ex. 10 12773 356 0.044 0.009 0.16 0.031 Com. Ex. 1116344 247 0.016 0.003 0.11 0.021 Com. Ex. 12 40896 713 0.101 0.020 0.230.045 Com. Ex. 13 31311 565 0.034 0.007 0.12 0.025 Com. Ex. 14 12130 3600.150 0.030 0.36 0.072 Com. Ex. 15 6386 583 0.000 0.000 0.01 0.002 Com.Ex. 16 27598 173 0.180 0.036 0.36 0.071 Com. Ex. 17 14208 328 0.1280.026 0.25 0.051 Com. Ex. 18 14625 271 0.004 0.001 0.04 0.008 Com. Ex.19 11412 363 0.051 0.010 0.18 0.035 Com. Ex. 20 26824 191 0.086 0.0170.19 0.037 Productivity Percentage of products Percentage Percentagewith High-temperature of products of products shrinkage characteristicsCasting with internal with misrun cavity Durability test yield defectsdefects defects No. evaluation evaluation (%) (%) (%) Present Ex. 1 GoodGood 1.0 0.3 0.8 Present Ex. 2 Good Good 2.2 0.1 1.2 Present Ex. 3 GoodGood 1.8 0.2 0.4 Present Ex. 4 Good Good 1.1 1.0 0.6 Present Ex. 5 GoodGood 1.4 0.4 1.6 Present Ex. 6 Good Acceptable 2.5 2.1 1.8 Present Ex. 7Good Acceptable 2.1 0.8 3.4 Com. Ex. 1 Poor Poor 2.8 0.2 9.5 Com. Ex. 2Good Poor 4.2 30.4 4.6 Com. Ex. 3 Acceptable Poor 2.6 38.6 12.2 Com. Ex.4 Poor Poor 5.5 0.7 20.1 Com. Ex. 5 Good Poor 1.2 25.1 1.3 Com. Ex. 6Acceptable Good 1.6 0.6 1.7 Com. Ex. 7 Acceptable Good 0.8 0.8 1.8 Com.Ex. 8 Poor Good 1.0 0.7 2.6 Com. Ex. 9 Poor Poor 3.5 36.0 1.0 Com. Ex.10 Poor Acceptable 2.1 2.3 1.1 Com. Ex. 11 Good Poor 1.6 47.1 7.1 Com.Ex. 12 Acceptable Acceptable 4.1 2.4 2.5 Com. Ex. 13 AcceptableAcceptable 2.6 2.3 3.3 Com. Ex. 14 Poor Poor 46.9 0.3 0.2 Com. Ex. 15Poor Poor 50.1 0.2 0.2 Com. Ex. 16 Poor Good 1.3 0.1 1.4 Com. Ex. 17Acceptable Poor 2.3 37.3 2.2 Com. Ex. 18 Acceptable Poor 39.3 3.7 0.2Com. Ex. 19 Poor Poor 21.6 0.3 1.4 Com. Ex. 20 Acceptable Poor 1.9 11.61.8

2. Measurement of Refining Agent Component Amounts

The refining agent content was measured at the end section of the discsection, and in sections other than the end section of the disc section.5 g of sample was collected for analysis from the end section 31 of thedisc section, the boss section 2, the vane section 4, and the discsection 32 other than the end section shown in FIG. 2, and the Ti and Bcontents were analyzed using an ICP emission spectrometer. The amountsof the refining agent components in sections other than the end sectionof the disc section given in Table 2 were calculated as mean values ofthe refining agent component amounts determined at the boss section, thevane sections, and the disc section other than the end section. Theresults are presented in Table 2.

3. High Temperature Characteristics

High-temperature fatigue strength was evaluated in a high-temperaturedurability test (turbo assembly; 150,000 rpm×200 h, outlet temperature200° C.). The results are presented in Table 2. The durability testevaluation results in Table 2 followed the following notation.

Poor: Fractured

Acceptable: No fracture, but cracking is occurred

Good: No fracture or cracking, and the sample remained intact

The parentheses following Acceptable and Poor indicate the location ofthe occurred cracks and fractures.

4. Casting Yield Evaluation

Casting yield was evaluated for 1,000 samples produced in each Example.Each sample was tested for external appearance failure due to misrunsand shrinkage cavity failure, and internal failure based on the detectedinternal blow holes in an X-ray examination. The proportions (%) ofsamples with misruns, shrinkage cavity failure, and internal failure inall samples were determined. The proportion (%) of non-defectiveproducts was then determined by subtracting the sum of the proportionsof these defective products from the total 100%. The results arepresented in Table 2.

Poor: The proportion of non-defective products is less than 90% (worsethan in existing products)

Acceptable: The proportion of non-defective products is 90% or more andless than 95% (same as in existing products)

Good: The proportion of non-defective products is 95% to 100% (greatimprovement over existing products)

In Present Examples 1 to 7, the surface density of intermetalliccompounds having a circle equivalent size of 1 to 6 μm at the endsection of the disc section, the surface density of intermetalliccompounds having a circle equivalent size of more than 6 μm at the endsection of the disc section, and the refining agent contents in the endsection of the disc section and in sections other than the end sectionfell in the specified ranges, and because of this, the high-temperaturecharacteristics, and the casting yield were both desirable.

In Comparative Example 1, with a high chill temperature, the surfacedensity was low in the intermetallic compounds having a circleequivalent size of 1 to 6 μm at the end section of the disc section, andwas high in the intermetallic compounds having a circle equivalent sizeof more than 6 μm at the end section of the disc section. Because ofthis, fracture occurred at the end section of the disc section, and thesample was inferior in terms of high-temperature characteristics.Further, multiple shrinkage cavity failures occurred at the bosssection, and the casting yield was considerably poor.

In Comparative Example 2, with a low chill temperature, multiple defectsoccurred in the appearance of the disc section due to misruns, and thecasting yield was poor.

In Comparative Example 3, there was a decrease in the molten metaltemperature. As a result, multiple defects occurred in the appearance ofthe vane sections due to misruns and shrinkage cavity, and the castingyield was considerably poor. Further, cracking occurred in the vanesections, and the high-temperature characteristics were poor.

In Comparative Example 4, the molten metal temperature was high, and thecooling rate was low. The surface density was therefore low in theintermetallic compounds having a circle equivalent size of 1 to 6 μm atthe end section of the disc section. As a result, multiple defectsoccurred in the appearance of the boss section due to shrinkage cavity,and the casting yield was considerably poor. Further, cracking occurredin the end section of the disc section, and the high-temperaturecharacteristics were poor.

In Comparative Example 5, with the low Cu content, the high-temperaturecharacteristics were desirable. However, there was a high incidence ofmisruns in the vane sections, and the casting yield was poor.

In Comparative Example 6, with the low Mg content, cracking occurred inthe boss section, and the high-temperature characteristics were poor.

In Comparative Example 7, with the low Fe content, cracking occurred inthe vane sections, and the high-temperature characteristics were poor.

In Comparative Example 8, with the low Ni content, fracture occurred inthe disc section, and the high-temperature characteristics were poor.

In Comparative Example 9, with the small amount of refining agentcomponent (B) in the molten metal preparation, the amounts of the Ti andB components became small in the end section of the disc section, and insections other than the end section of the disc section. The grainrefining effect was therefore insufficient, and the surface density waslow in the intermetallic compounds having a circle equivalent size of 1to 6 μm in the end section of the disc section, and was high in theintermetallic compounds having a circle equivalent size of more than 6μm. As a result, multiple defects occurred in the appearance of the vanesections due to misruns, and the casting yield was considerably poor.Further, fracture occurred in the disc section, and the high-temperaturecharacteristics were poor.

In Comparative Example 10, with the high Cu content, fracture occurredin the disc section, and high-temperature characteristics were poor.

In Comparative Example 11, with the high Mg content, multiple misrunsoccurred in the vane sections, and the casting yield was poor, thoughthe high-temperature characteristics were desirable.

In Comparative Example 12, with the high Fe content, the intermetalliccompounds having a circle equivalent size of more than 6 μm had a highsurface density. Because of this, cracking occurred in the disc section,and the high-temperature characteristics were poor.

In Comparative Example 13, with the high Ni content, the intermetalliccompounds having a circle equivalent size of more than 6 μm had a highsurface density. Because of this, cracking occurred in the disc section,and the high-temperature characteristics were poor.

In Comparative Example 14, the amounts of the Ti and B components werehigh in sections other than the end section of the disc section becauseof the large amounts of the Ti and B components in the molten metalpreparation, and the high pour rate of molten metal into the space mold(here and below, “space mold” refers to the product-shape spaceconfigured from the plaster mold and the chill). Because of this,fracture occurred in the boss section, and the high-temperaturecharacteristics were poor. There was also a disturbed molten metal flowin the space mold, and the casting yield was considerably poor becauseof multiple internal failures.

In Comparative Example 15, the amounts of the Ti and B components in theend section of the disc section were small (0%) because of the high pourrate of molten metal into the space mold, though the amounts of the Tiand B components in the molten metal preparation were in the specifiedranges. Because of this, the grain refining effect was insufficient, andthe surface density was low in the intermetallic compounds having acircle equivalent size of 1 to 6 μm in the end section of the discsection, and was high in the intermetallic compounds having a circleequivalent size of more than 6 μm. As a result, fracture occurred in thedisc section, and the high-temperature characteristics were poor. Therewas also a disturbed molten metal flow in the space mold, and thecasting yield was considerably poor because of multiple internalfailures.

In Comparative Example 16, because of the large amounts of the Ti and Bcomponents in the molten metal preparation, the amounts of the Ti and Bcomponents were large in the end section of the disc section, and insections other than the end section of the disc section. This causedaggregation of refining agent particles. Further, fracture occurred inthe disc section, and the high-temperature characteristics were poor.

In Comparative Example 17, because of the slow pour rate of molten metalinto the plaster mold, the molten metal temperature decreased in theprocess of delivering the molten metal to the plaster mold. This causedmultiple failures in the appearance of the vane sections due to misruns,and the casting yield was considerably poor. Further, cracking occurredin the vane sections, and the high-temperature characteristics werepoor.

In Comparative Example 18, because of the high pour rate of molten metalinto the plaster mold, the inertial movement of the refining agentparticles had difficulties following the pour rate of the molten metal.Because of this, the Ti content was low in the end section of the discsection, and the refining agent particles failed to reach the endsection of the disc section in sufficient amounts. Refining of crystalgrains therefore did not take place, and coarse intermetallic compoundswere created. This caused cracking in the end section of the discsection, and the high-temperature characteristics were poor. There wasalso a disturbed molten metal flow in the space mold, and the castingyield was considerably poor because of multiple internal failures.

In Comparative Example 19, because of the lateral pour direction ofmolten metal into the space mold, nonuniform solidification occurred inthe circumferential direction of the compressor impeller. This causedcracking in the boss section due to an axial runout, and thehigh-temperature characteristics were poor. Further, because the moltenmetal was nonuniformly charged into the space mold, multiple internalfailures occurred, and the casting yield was considerably poor.

In Comparative Example 20, because of the downward pour direction ofmolten metal into the space mold, the molten metal temperature decreasedin the process of delivering the molten metal to the plaster mold. Thiscaused multiple failures in the appearance of the disc section due tomisruns, and the casting yield was considerably poor. Further, crackingoccurred in the disc section, and the high-temperature characteristicswere poor.

Second Example (Present Examples 8 to 18, and Comparative Examples 21 to26)

Each Al alloy of the composition shown under the column “Components” inTable 3 was melted by using a common molten metal process, and themolten metal was adjusted to the temperature shown in Table 3 by amolten metal preparation step. In the molten metal preparation step, 150kg of the Al alloy of the composition shown under the column“Components” in Table 3 was melted to obtain a molten metal. Thereafter,a blow degassing process was performed by blowing argon gas into themolten metal for 20 minutes with a rotary gas blower operated at arotation speed of 400 rpm, and a gas flow rate of 2.5 Nm³/h. The wholemolten metal was held still for 1 hour to remove the slag. After theslag removal, a refining agent was added to the molten metal in themetal preparation step to make the amounts of the Ti and B components asshown under the column “Amounts of refining agent components aftermolten metal preparation” in Table 3.

TABLE 3 Amounts of refining agent Heat treatment conditions Castingconditions components after Solution Molten molten metal treatment Agingtreatment Components (mass %) metal temp. Chill temp. preparation (mass%) temp. × time temp. × time No. Cu Mg Ni Fe Al (° C.) (° C.) Ti B (° C.× hour) (° C. × hour) Present Ex. 8 2.6 1.6 1.1 0.9 Remainder 780 2000.15 0.03 525 × 5 190 × 16 Present Ex. 9 excluding Ti 760 170 515 × 10190 × 24 Present Ex. 10 and B 770 240 515 × 10 190 × 24 Present Ex. 11720 150 530 × 4 230 × 9 Present Ex. 12 740 160 505 × 10 230 × 9 PresentEx. 13 740 140 535 × 2 230 × 9 Present Ex. 14 750 210 520 × 8 200 × 2Present Ex. 15 750 220 520 × 8 200 × 34 Present Ex. 16 740 180 520 × 8170 × 24 Present Ex. 17 740 200 520 × 8 240 × 24 Present Ex. 18 760 110525 × 1 200 × 20 Com. Ex. 21 715 200 520 × 6 200 × 16 Com. Ex. 22 785220 520 × 6 200 × 16 Com. Ex. 23 740 90 520 × 6 200 × 16 Com. Ex. 24 750260 520 × 6 200 × 16 Com. Ex. 25 760 190 None 190 × 24 Com. Ex. 26 740200 530 × 6 None

The Al alloy molten metal prepared in the molten metal preparation stepwas then subjected to low-pressure casting to produce an Al alloycasting, whereby the molten metal was pressure injected into apredetermined space configured from a plaster mold that had beenadjusted to 220° C., and a copper chill that had been adjusted to thetemperature shown in Table 3 and disposed on the surface that contactsthe impeller disc surface. The Al alloy casting was intended as aturbocharger compressor impeller for trucks, and had a shape with a discsection measuring 80 mm in diameter, a boss section measuring 70 mm inheight, vane sections measuring 60 mm in height and having 14 vanes thatwere 0.4 mm in thickness at the vane tip. As shown in FIG. 3, the spaceconfigured from the plaster mold and the chill was formed by verticallydisposing the plaster mold and the chill so that the chill was higher inposition than the plaster mold having a molten metal inlet at thebottom. The pour direction of molten metal was upward. The molten metalwas pressure injected into the space at a pour rate of 0.75 m/s at themolten metal inlet, and the pressure was maintained until the whole Alalloy casting solidified.

The Al alloy casting was removed from the plaster mold, and subjected toa solution treatment under the conditions shown in Table 3, andthereafter an aging treatment under the conditions of Table 3. In thisway, a sample Al alloy cast impeller for compressors was prepared.

The samples produced in the manner described above were each evaluatedin the same manner as in First Example with respect to the surfacedensity of intermetallic compounds having a circle equivalent size of 1to 6 μm at the end section of the disc section, the surface density ofintermetallic compounds having a circle equivalent size of more than 6μm at the end section of the disc section, amounts of the refining agentcomponents (Ti, B) at the end section of the disc section, amounts ofthe refining agent components (Ti, B) in sections other than the endsection of the disc section, high-temperature characteristics(durability test evaluation), and productivity (casting yieldevaluation). The results are presented in Table 4. For high-temperaturecharacteristics, samples that scored “Good” in the evaluation of FirstExample were further tested for 100 hours for a total of 300 hours underthe same conditions (turbo assembly, 150000 rpm, output temperature 200°C.). Samples that produced desirable results after the 300-hour testwere evaluated as “Excellent.”

TABLE 4 Amounts of Amounts of refining agent Intermetallic compounds atend refining agent components in section of disc section components atsections other Surface density of Surface density of end section of thanend section intermetallic compounds intermetallic compounds disc sectionof disc section having circle equivalent having circle equivalent Ti BTi B size of 1 to 6 μm size of more than 6 μm content content contentcontent (Number of intermetallic (Number of intermetallic (mass (mass(mass (mass No. compounds/mm²) compounds/mm²) %) %) %) %) Present Ex. 815576 284 0.048 0.010 0.12 0.025 Present Ex. 9 11382 410 0.018 0.0040.09 0.019 Present Ex. 10 12225 387 0.066 0.013 0.14 0.028 Present Ex.11 11837 404 0.011 0.002 0.09 0.017 Present Ex. 12 15759 252 0.024 0.0050.10 0.020 Present Ex. 13 12905 359 0.012 0.002 0.09 0.017 Present Ex.14 13780 317 0.053 0.011 0.13 0.026 Present Ex. 15 15647 283 0.057 0.0110.13 0.026 Present Ex. 16 10027 424 0.024 0.005 0.10 0.020 Present Ex.17 16027 235 0.038 0.008 0.11 0.023 Present Ex. 18 15633 303 0.023 0.0040.09 0.019 Com. Ex. 21 14007 299 0.032 0.006 0.11 0.021 Com. Ex. 22 6448766 0.063 0.013 0.14 0.028 Com. Ex. 23 13743 332 0.009 0.002 0.08 0.017Com. Ex. 24 8335 549 0.075 0.015 0.15 0.030 Com. Ex. 25 16103 232 0.0320.006 0.11 0.021 Com. Ex. 26 16187 231 0.045 0.009 0.12 0.024Productivity Percentage Percentage of products of products Percentagewith High-temperature with of products shrinkage characteristics Castinginternal with misrun cavity Durability test yield defects defectsdefects No. evaluation evaluation (%) (%) (%) Present Ex. 8 ExcellentGood 1.3 0.4 1.1 Present Ex. 9 Excellent Good 1.9 0.2 2.0 Present Ex. 10Excellent Good 1.5 0.3 2.3 Present Ex. 11 Excellent Good 2.3 0.8 1.2Present Ex. 12 Good Good 2.1 0.9 1.2 Present Ex. 13 Good Good 1.3 1.11.3 Present Ex. 14 Good Good 1.8 0.4 2.1 Present Ex. 15 Good Good 1.60.2 2.3 Present Ex. 16 Good Good 1.5 1.2 1.5 Present Ex. 17 Good Good2.1 0.3 2.1 Present Ex. 18 Good Good 1.2 0.6 2.1 Com. Ex. 21 AcceptablePoor 6.3 26.3 2.3 Com. Ex. 22 Poor Poor 2.5 1.2 17.3 Com. Ex. 23Acceptable Poor 1.5 38.1 4.1 Com. Ex. 24 Poor Acceptable 2.0 1.1 5.2Com. Ex. 25 Poor Good 1.6 0.3 1.5 Com. Ex. 26 Poor Good 1.1 1.0 1.2

In Present Examples 8 to 18, the surface density of intermetalliccompounds having a circle equivalent size of 1 to 6 μm at the endsection of the disc section, the surface density of intermetalliccompounds having a circle equivalent size of more than 6 μm at the endsection of the disc section, and the amounts of the refining agentcomponents in the end section of the disc section and in sections otherthan the end section fell in the specified ranges, and because of this,the high-temperature characteristics, and the casting yield were bothdesirable.

On the other hand, in Comparative Example 21, the molten metaltemperature was low, and the casting yield was poor with multiplefailures occurring in the appearance of the vane sections due tomisruns. Further, cracking occurred in the vane sections, and thehigh-temperature characteristics were poor.

In Comparative Example 22, the molten metal temperature was high, andthe cooling rate was low. The surface density was therefore low in theintermetallic compounds having a circle equivalent size of 1 to 6 μm atthe end section of the disc section, and was high in the intermetalliccompounds having a circle equivalent size of more than 6 μm at the endsection of the disc section. This caused multiple failures in theappearance of the boss section due to shrinkage cavity, and the castingyield was considerably poor. Further, fracture occurred in the discsection, and the high-temperature characteristics were poor.

In Comparative Example 23, with a low chill temperature, multiplemisruns occurred in the disc section, and the casting yield was poor.Further, cracking due to misruns occurred in the disc, and thehigh-temperature characteristics were poor.

In Comparative Example 24, the chill temperature was high, and thesurface density was low in the intermetallic compounds having a circleequivalent size of 1 to 6 μm at the end section of the disc section, andwas high in the intermetallic compounds having a circle equivalent sizeof more than 6 μm at the end section of the disc section. This causedfractures in the disc section, and the high-temperature characteristicswere poor.

The solution treatment step was not performed in Comparative Example 25.The aging treatment step was not performed in Comparative Example 26. Asa result, fracture occurred in the disc section, and thehigh-temperature characteristics were poor.

Third Example (Present Examples 19 to 28, and Comparative Examples 27 to32)

Each Al alloy of the composition shown under the column “Components” inTable 5 was melted by using a common molten metal process, and themolten metal was adjusted to the temperature shown in Table 5 by amolten metal preparation step. In the molten metal preparation step, 200kg of the Al alloy of the composition shown under the column“Components” in Table 5 was melted to obtain a molten metal. Thereafter,a blow degassing process was performed by blowing argon gas into themolten metal for 40 minutes with a rotary gas blower operated at arotation speed of 400 rpm, and a gas flow rate of 2.5 Nm³/h. The wholemolten metal was held still for 1.5 hours to remove the slag. After theslag removal, a refining agent was added to the molten metal in themetal preparation step to make the amounts of the Ti and B components asshown under the column “Amounts of refining agent components aftermolten metal preparation” in Table 5.

TABLE 5 Amounts of refining agent Heat treatment conditions componentsafter Solution Casting conditions molten metal treatment Aging treatmentComponents (mass %) Molten metal Chill temp. preparation (mass %) temp.× time temp. × time No. Cu Mg Ni Fe Al temp. (° C.) (° C.) Ti B (° C. ×hour) (° C. × hour) Present Ex. 19 2.9 1.7 1.1 1.1 Remainder 770 2000.17 0.02 525 × 6 190 × 22 Present Ex. 20 excluding 780 230 510 × 8 190× 22 Present Ex. 21 Ti and B 760 240 515 × 10 190 × 22 Present Ex. 22740 190 530 × 4 200 × 12 Present Ex. 23 760 180 505 × 8 200 × 12 PresentEx. 24 730 150 535 × 3 200 × 12 Present Ex. 25 750 220 515 × 8 220 × 2Present Ex. 26 720 250 515 × 8 220 × 32 Present Ex. 27 730 120 515 × 8175 × 24 Present Ex. 28 720 100 515 × 8 235 × 20 Com. Ex. 27 785 200 530× 4 195 × 18 Com. Ex. 28 715 180 530 × 4 195 × 18 Com. Ex. 29 740 95 530× 4 195 × 18 Com. Ex. 30 750 255 530 × 4 195 × 18 Com. Ex. 31 760 180None 195 × 18 Com. Ex. 32 750 210 530 × 4 None

The Al alloy molten metal prepared in the molten metal preparation stepwas then subjected to low-pressure casting to produce an Al alloycasting, whereby the molten metal was pressure injected into apredetermined space configured from a plaster mold that had beenadjusted to 220° C., and a copper chill that had been adjusted to thetemperature shown in Table 5 and disposed on the surface that contactsthe impeller disc surface. The Al alloy casting was intended as aturbocharger compressor impeller for ships, and had a shape with a discsection measuring 150 mm in diameter, a boss section measuring 160 mm inheight, vane sections measuring 120 mm in height and having 16 vanesthat were 0.6 mm in thickness at the vane tip. As shown in FIG. 3, thespace configured from the plaster mold and the chill was formed byvertically disposing the plaster mold and the chill so that the chillwas higher in position than the plaster mold having a molten metal inletat the bottom. The pour direction of molten metal was upward. The moltenmetal was pressure injected into the space at a pour rate of 0.95 m/s atthe molten metal inlet, and the pressure was maintained until the wholeAl alloy casting solidified.

The Al alloy casting was removed from the plaster mold, and subjected toa solution treatment under the conditions shown in Table 5, andthereafter to an aging treatment under the conditions of Table 5. Inthis way, a sample Al alloy cast impeller for compressors was prepared.

The samples produced in the manner described above were each evaluatedin the same manner as in First Example with respect to the surfacedensity of intermetallic compounds having a circle equivalent size of 1to 6 μm at the end section of the disc section, the surface density ofintermetallic compounds having a circle equivalent size of more than 6μm at the end section of the disc section, amounts of the refining agentcomponents (Ti, B) at the end section of the disc section, amounts ofthe refining agent components (Ti, B) in sections other than the endsection of the disc section, high-temperature characteristics(durability test evaluation), and productivity (casting yieldevaluation). The results are presented in Table 6.

TABLE 6 Intermetallic compounds at end section of disc section Surfacedensity of Surface density of Amounts of refining intermetallicintermetallic Amounts of refining agent components compounds havingcompounds having agent components in sections other circle equivalentsize circle equivalent size at end section of than end section of of 1to 6 μm of more than 6 μm disc section disc section (Number of (Numberof Ti Ti intermetallic intermetallic content B content content B contentNo. compounds/mm²) compounds/mm²) (mass %) (mass %) (mass %) (mass %)Present Ex. 19 11759 207 0.022 0.004 0.07 0.014 Present Ex. 20 11460 2450.034 0.007 0.08 0.017 Present Ex. 21 11769 236 0.037 0.007 0.09 0.017Present Ex. 22 13697 164 0.022 0.004 0.07 0.014 Present Ex. 23 12648 1770.019 0.004 0.07 0.014 Present Ex. 24 13975 153 0.015 0.003 0.07 0.013Present Ex. 25 10856 362 0.032 0.006 0.08 0.016 Present Ex. 26 13036 1700.045 0.009 0.10 0.019 Present Ex. 27 11416 255 0.014 0.003 0.06 0.013Present Ex. 28 11316 301 0.013 0.003 0.06 0.013 Com. Ex. 27 7235 6420.022 0.004 0.07 0.014 Com. Ex. 28 12243 205 0.015 0.003 0.07 0.013 Com.Ex. 29 10490 420 0.006 0.001 0.06 0.011 Com. Ex. 30 9244 524 0.039 0.0080.09 0.018 Com. Ex. 31 10506 494 0.018 0.004 0.07 0.014 Com. Ex. 3212598 186 0.029 0.006 0.08 0.016 Productivity Percentage of productsPercentage Percentage with High-temperature of products of productsshrinkage characteristics Casting with internal with misrun cavityDurability test yield defects defects defects No. evaluation evaluation(%) (%) (%) Present Ex. 19 Excellent Good 1.6 0.4 1.2 Present Ex. 20Excellent Good 1.2 0.3 2.2 Present Ex. 21 Excellent Good 1.4 0.4 2.1Present Ex. 22 Excellent Good 1.9 0.6 1.6 Present Ex. 23 Good Good 2.50.7 1.0 Present Ex. 24 Good Good 1.5 1.0 1.6 Present Ex. 25 Good Good1.1 0.5 2.3 Present Ex. 26 Good Good 1.8 0.4 2.1 Present Ex. 27 GoodGood 1.4 1.1 2.0 Present Ex. 28 Good Good 2.2 0.4 1.8 Com. Ex. 27 PoorPoor 5.5 0.8 31.8 Com. Ex. 28 Acceptable Poor 3.1 60.1 1.8 Com. Ex. 29Acceptable Poor 2.2 41.4 3.7 Com. Ex. 30 Poor Acceptable 3.0 1.0 4.8Com. Ex. 31 Poor Good 1.3 0.4 1.1 Com. Ex. 32 Poor Good 1.5 0.8 1.0

In Present Examples 19 to 28, the surface density of intermetalliccompounds having a circle equivalent size of 1 to 6 μm at the endsection of the disc section, the surface density of intermetalliccompounds having a circle equivalent size of more than 6 at the endsection of the disc section, and the amounts of the refining agentcomponents in the end section of the disc section and in sections otherthan the end section fell in the specified ranges, and thehigh-temperature characteristics, and the casting yield were bothdesirable.

On the other hand, in Comparative Example 27, the molten metaltemperature was high, and the cooling rate was low. The surface densitywas therefore low in the intermetallic compounds having a circleequivalent size of 1 to 6 μm at the end section of the disc section, andwas high in the intermetallic compounds having a circle equivalent sizeof more than 6 μm at the end section of the disc section. This causedmultiple failures in the appearance of the boss section due to shrinkagecavity, and the casting yield was considerably poor. Further, fractureoccurred in the disc section, and the high-temperature characteristicswere poor.

In Comparative Example 28, the molten metal temperature was low, and thecasting yield was poor with multiple failures occurring in theappearance of the vane sections due to misruns. Further, crackingoccurred in the vane sections, and the high-temperature characteristicswere poor.

In Comparative Example 29, with the low chill temperature, multiplemisruns occurred in the disc section, and the casting yield was poor.Further, cracking due to misruns occurred in the disc, and thehigh-temperature characteristics were poor.

In Comparative Example 30, the chill temperature was high, and thesurface density was low in the intermetallic compounds having a circleequivalent size of 1 to 6 μm at the end section of the disc section, andwas high in the intermetallic compounds having a circle equivalent sizeof more than 6 μm at the end section of the disc section. This causedfractures in the disc section, and the high-temperature characteristicswere poor.

The solution treatment step was not performed in Comparative Example 31,and the aging treatment step was not performed in Comparative Example32. As a result, the disc section was damaged, and high-temperaturecharacteristics was poor.

INDUSTRIAL APPLICABILITY

The present invention enables inexpensively providing an Al alloyimpeller for compressors that has excellent high-temperature strength,and that can stably withstand an increasing of temperatures due to anincreasing of number of rotations over extended time periods. Thepresent invention is also industrially very effective in that the outputpower of an internal combustion engine can be improved by increasing thesupercharge ability of a turbocharger.

DESCRIPTION OF REFERENCE SIGNS

-   -   1 Compressor impeller    -   2 Boss section    -   3 Disc section    -   31 End section of disc section    -   32 Disc section excluding end section    -   4 Vane section    -   5 Central axis    -   6 Chill    -   7 Plaster mold    -   8 Molten metal inlet    -   9 Molten metal pour direction    -   10 Product-shape space configured from plaster mold and chill

1. A compressor impeller cast from an Al alloy, comprising: a bosssection; a plurality of vane sections; and a disc section, wherein theboss section, the plurality of vane sections, and the disc sectionexcluding an end section comprise an Al alloy comprising Cu: 1.4 to 3.2mass %, Mg: 1.0 to 2.0 mass %, Ni: 0.5 to 2.0 mass %, Fe: 0.5 to 2.0mass %, Ti: 0.01 to 0.35 mass %, and B: 0.002 to 0.070 mass % and abalance of Al and unavoidable impurities, wherein the end section of thedisc section comprises an Al alloy comprising Cu: 1.4 to 3.2 mass %, Mg:1.0 to 2.0 mass %, Ni: 0.5 to 2.0 mass %, Fe: 0.5 to 2.0 mass %, Ti:0.005 to 0.175 mass %, and B: 0.001 to 0.035 mass % and a balance of Aland unavoidable impurities, and wherein at least 10000/mm² ofintermetallic compounds having a circle-equivalent diameter of 1 to 6μm, and no greater than 500/mm² of intermetallic compounds having acircle-equivalent diameter exceeding 6 μm exist in the end section ofthe disc section.
 2. The compressor impeller cast from the Al alloyaccording to claim 1, wherein the compressor impeller is for use inlarge-scale applications including ships, and wherein the boss sectionhas a height of 200 to 80 mm, the disc section has a diameter of 300 to100 mm and the vane sections have a height of 180 to 60 mm with 30 to 10vanes measuring 4.0 to 0.4 mm in thickness at a vane tip.
 3. Thecompressor impeller cast from the Al alloy according to claim 1, whereinthe compressor impeller is for use in small-scale applications includingautomobiles, and wherein the boss section has a height of 100 to 20 mm,the disc section has a diameter of 120 to 25 mm, and the vane sectionshave a height of 90 to 5 mm with 20 to 4 vanes measuring 3.0 to 0.1 mmin thickness at a vane tip.
 4. A method for producing a compressorimpeller cast from an Al alloy, comprising: a molten metal preparationstep of preparing a 720 to 780° C. Al alloy molten metal comprising Cu:1.4 to 3.2 mass %, Mg: 1.0 to 2.0 mass %, Ni: 0.5 to 2.0 mass %, Fe: 0.5to 2.0 mass % and a balance of Al and unavoidable impurities, and addinga refining agent to the Al alloy molten metal to incorporate Ti: 0.01 to0.35 mass % and B: 0.002 to 0.070 mass % in an alloy composition of theAl alloy molten metal; a casting step of casting an Al alloy casting bypressure casting whereby the Al alloy molten metal prepared is injectedthrough a molten metal inlet into a space having a product shapeconfigured from a plaster mold having the molten metal inlet at thebottom of the plaster mold, and a 100 to 250° C. chill disposed on asurface that contacts with an impeller disc surface, the space beingformed by disposing the plaster mold and the chill so that the chill isat upper position and the plaster mold is at below position, and aninflow rate at the molten metal inlet into the space being 0.12 to 1.00m/s; a solution treatment step of solution treating by subjecting the Alalloy casting to a solution treatment; and an aging treatment step ofaging treating by subjecting the Al alloy casting to aging after thesolution treatment.
 5. The method for producing the compressor impellercast from the Al alloy according to claim 4, wherein an end section ofan disc section of the compressor impeller has a cooling rate of 0.1 to200° C./s in the casting step.
 6. The method for producing thecompressor impeller cast from the Al alloy according to claim 4, whereinthe Al alloy casting is heat treated for 2 hours or more at atemperature 5 to 25° C. below a solidus temperature of the Al alloy inthe solution treatment step, and wherein the Al alloy casting after thesolution treatment step is subjected to a heat treatment at 180 to 230°C. for 3 to 30 hours in the aging treatment step.
 7. The method forproducing the compressor impeller cast from the Al alloy according toclaim 5, wherein the Al alloy casting is heat treated for 2 hours ormore at a temperature 5 to 25° C. below a solidus temperature of the Alalloy in the solution treatment step, and wherein the Al alloy castingafter the solution treatment step is subjected to a heat treatment at180 to 230° C. for 3 to 30 hours in the aging treatment step.